Germany's interior minister said on Sunday there were "clear limits" if Turkish ministers want to campaign among immigrants in Germany, striking a tougher tone than the previous line from Berlin as a row between Ankara and EU states escalates, Reuters reported.
Thomas de Maiziere told broadcaster ARD he did not support ministers campaigning in Germany.
"You have to carefully consider whether one imposes a ban on entry. But there are limits - clear limits - for example the criminal code: we have provisions in our criminal code," de Maiziere told broadcaster ARD.
"Anyone who insults Germany or its constitutional order and derides it in a malicious way is liable to prosecution. That, at least, would be a limit."
On Saturday Turkish President Tayyip Erdogan branded the Netherlands "Nazi remnants, fascists" after the Dutch government withdrew permission for his foreign minister to land there.
Berlin has said Turkey must not use Nazi comparisons but has stopped short of advocating a ban on ministers campaigning for a referendum next month on whether to grant the president greater powers.
Individual local authorities last week canceled several planned appearances due to security concerns but the foreign minister made a speech in Hamburg.
Turkey told the Netherlands on Sunday that it would retaliate after its ministers were barred from speaking in Rotterdam.